(CNN) -- The officer involved in a New Year's Day shooting that left a passenger dead in a crowded Oakland, California, subway station resigned Wednesday, a Bay Area Rapid Transit spokesman said.

Oscar Grant, 22, was killed January 1 in a shooting at a subway station in California's Bay Area.

BART spokesman Linton Johnson said in a statement that the officer's attorney and his union representative submitted the resignation at a meeting Wednesday morning with BART police investigators.

The resignation was effective immediately.

In the statement, Johnson said the officer's attorney had postponed a meeting with investigators Monday and on Tuesday asked for a delay until next week. That request was denied, and the Wednesday meeting was scheduled. The officer did not show, Johnson said.

Shortly before the funeral for the passenger, 22-year-old Oscar Grant, on Wednesday, Johnson said Mehserle had been moved at least twice because of death threats to him and his family.

The BART officer fatally shot Grant in the back after he and some friends were pulled from a train car following a report of an altercation, according to a BART statement.

On Tuesday, attorney John Burris filed a $25 million claim with BART in which he identified the officer as Johannes Mehserle. Several local media outlets had already identified the officer.

Burris alleges in the claim, "Without so much as flinching the Officer Mehserle stood over Grant and mercilessly fired his weapon, mortally wounding Mr. Grant with a single gunshot wound to the back." Watch the events preceding the shooting »

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Johnson said Mehserle's attorney -- reportedly David E. Mastagni of Sacramento -- has advised him not to speak to investigators. Nor has Mehserle made any public statements.

An attorney in Mastagni's office, Christopher Miller, confirmed the resignation Wednesday, saying Mehserle had the support of the BART Police Officers' Association.

CNN affiliate KTVU-TV in Oakland obtained videos of the incident and its prelude. One video, which KTVU reported came from a train passenger who wished not to be identified, shows three young men against a wall in the Fruitvale station.

Burris told CNN on Tuesday that the young men had been celebrating the new year at a popular waterfront tourist spot, The Embarcadero. They were heading home when police pulled them from the train car about 2 a.m.

Some of the young men were handcuffed, but not 22-year-old Oscar Grant. The video from the anonymous passenger shows Grant seated on the floor with his back against the wall. iReport.com: BART protests turn violent

Grant holds up his hands, appearing to plead with police. Burris said Tuesday that Grant was asking police not to use a Taser.

"He said to them, 'Don't Tase me; I have a 4-year-old daughter,' " Burris said.

The interaction on the video is not audible.

Seconds later, police put Grant face-down on the ground. Grant appears to struggle. One of the officers kneels on Grant as another officer stands, tugs at his gun, unholsters it and fires a shot into Grant's back.

Burris said the bullet went through Grant's back and then ricocheted off the floor and through his lungs.

Grant died seven hours later at a hospital, KTVU reported.

In Wednesday's statement about Mehserle's resignation, BART General Manager Dorothy Dugger said that the "shooting is a tragic event in every respect for all involved."

"We recognize that the family and friends of Oscar Grant are in mourning, and we extend our condolences," she said.

Johnson has said Mehserle, who had been on the force two years, is devastated and is presumed innocent. He also says that the videos making the media rounds are inconclusive and that there is more to the story than what can be seen on the grainy images.

There are two surveillance cameras at the Fruitvale station, but a BART official said Tuesday that no video is being released at this time.

Burris called the shooting "unconscionable" and said the $25 million claim alleges wrongful death and violation of civil rights by use of excessive force. Read the claim (PDF)

BART has 45 days to respond, he said. If the authority rejects the claim, he will file a civil lawsuit, said Burris, who served as Rodney King's co-counsel in King's civil case against the Los Angeles Police Department in the 1990s.

Burris has spoken to witnesses who claim that Grant was trying to resolve the situation.

"He had been telling people to calm down. 'Be cool. Just do what they tell you to do,' " the attorney said.

Johnson told KTVU that authorities are trying to determine whether Mehserle accidentally drew his gun instead of his Taser.

Burris said he is pushing Alameda County District Attorney Tom Orloff to press second-degree murder charges, or involuntary manslaughter charges if evidence suggests that Mehserle mistook his gun for a Taser, he said.

"No one wants to believe a cop would just kill somebody like that," he said. "My view is, this is criminal conduct, period."

Orloff said Wednesday that his investigation will focus primarily on what led to the shooting.

Some homicides are lawful, he noted. In this case, Orloff said, "the part that needs dissecting is what, if anything, can be determined about the mental state of the actor," meaning the officer.

There are many possibilities, he said: His office could find no basis for criminal charges; the office could file involuntary manslaughter charges if Mehserle exercised gross negligence, voluntary manslaughter if Mehserle reasonably believed that he was acting in self-defense or murder if Mehserle acted with malice and forethought.

"Our function is to determine whether or not criminal charges should be filed against the officer," he said. "These things are usually an issue of weeks rather than days."

BART Police Chief Gary Gee released a statement this week expressing condolences for Grant's family and saying the authority is cooperating with Orloff's office.

Gee added that BART will complete an "unbiased and thorough investigation" and asked the public to be patient.

"As frustrating as it is, I want to stress that we cannot and will not jeopardize this case by discussing details before the investigation is complete," Gee said.

Johnson told CNN on Wednesday that it could take weeks for BART to complete its investigation.